1. Field of the Invention
The present invention relates to an alternator comprising a rectifier cooled by external air drawn in through openings in a bracket by the rotation of a fan secured to a rotor.
2. Description of the Related Art
FIG. 9 is a cross section of a conventional automotive alternator, FIG. 10 is a perspective of the rectifier in FIG. 9, and FIG. 11 is a perspective of the bearing housing portion in FIG. 9.
This automotive alternator includes: a case 3 comprising an aluminum rear bracket 1 and an aluminum front bracket 2; a shaft 6 rotatably supported by a first bearing 31 inserted into a bearing housing portion 30 in the rear bracket 1 which constitutes a first bracket and a second bearing 33 inserted into a bearing housing portion 32 in the front bracket 2 which constitutes a second bracket, a pulley 4 being secured to one end of the shaft 6; a Lundell-type rotor 7 secured to the shaft 6; a stator 8 secured to an inner wall of the case 3; slip rings 9 secured to the other end of the shaft 6 for supplying electric current to the rotor 7; a pair of brushes 10 moving in contact with the slip rings 9; a brush holder 11 accommodating the brushes 10; a rectifier 12 electrically connected to the stator 8 for converting alternating current generated in the stator 8 into direct current; a heat sink 17 fitted over the brush holder 11; and a regulator 18 attached to the heat sink by adhesive for adjusting the alternating current generated in the stator 8.
The rotor 7 includes: a rotor coil 13 for generating magnetic flux by passing electric current therethrough; and a pole core 14 disposed so as to cover the rotor coil 13 in which magnetic poles are produced by the magnetic flux generated by the rotor coil 13. The pole core 14 includes a first pole core assembly 21 and a second pole core assembly 22 which mutually interlock. Centrifugal fans 5 for cooling are welded to the axial ends of the first pole core assembly 21 and second pole core assembly 22.
The stator 8 includes: a stator core 15; and a stator coil 16 composed of wire wound onto the stator core 15 in which alternating current is generated by changes in the magnetic flux from the rotor coil 13 as the rotor 7 rotates.
The rectifier 12 includes: a positive-side heat sink 24 comprising an arc-shaped heat sink main body 24a and a plurality of fins 24b standing on the reverse side of the heat sink main body 24a; four positive-side diodes 23 composed of molded resin secured by soldering to the upper surface of the positive-side heat sink main body 24a; an arc-shaped negative-side heat sink 26 grounded by being secured to the rear bracket 1; four negative-side diodes 25 composed of molded resin secured by soldering to the negative-side heat sink 26; and a circuit board 27 for electrically connecting each of the diodes 23 and 25 to the stator coil 16, the rectifier 12 converting the three-phase alternating current generated by the stator 8 into a direct current.
The positive-side heat sink 24 and the negative-side heat sink 26 are disposed on a generally flat plane in the radial direction of the shaft 6, and are housed inside the case 3. The positive-side heat sink 24 and the negative-side heat sink 26 are composed of aluminum which has high thermal conductivity.
In a vehicle alternator of the above construction, a current is supplied by a battery (not shown) through the brushes 10 and slip rings 9 to the rotor coil 13, whereby magnetic flux is generated, giving rise to a magnetic field and at the same time, the pulley 4 is driven by the engine and the rotor 7 is rotated by the shaft 6, so that a rotating magnetic field is imparted to the stator coil 16 and electromotive force is generated in the stator coil 16. This alternating electromotive force passes through the positive-side diodes 23 and the negative-side diodes 25 of the rectifier 12 and is converted into direct current, the magnitude thereof is adjusted by the regulator 18, and the battery is recharged.
While the alternator is generating power, the rotor coil 13, the stator coil 16, the positive-side diodes 23, the negative-side diodes 25, and the regulator 18 are constantly generating heat. For example, in an alternator with a rated output current in the 100 A class, the amount of heat generated is 60 W in the rotor coil 13, 500 W in the stator coil 16, a total of 120 W in the positive-side diodes 23 and the negative-side diodes 25, and 6 W in the regulator 18. The excessive generation of heat causes deterioration in the performance of the alternator and reduces the working life of the parts.
For that reason, the fans 5 are rotated together with the rotation of the rotor 7, external air introduced into the case 3 from openings A1 and A2 in the case 3 by this rotation flows as indicated by the arrows .alpha. and .beta. in FIG. 9, mainly cooling the positive-side heat sink 24 and the positive-side diodes 23. The external air then flows radially outwards from the fans 5, cools the end portions of the stator coil 16 in the rear end, and is expelled to the outside through openings B.
External air is also introduced into the case 3 from openings C by the rotation of the fans 5, and the external air flows as indicated by the arrow .gamma. in FIG. 9, cooling the power transistors of the regulator 18. The external air then flows radially outwards from the fans 5, cools the end portions of the stator coil 16 in the rear end, and is expelled to the outside through openings D.
Similarly, external air introduced through openings E in the front bracket 2 flows radially outwards from the fans 5, cooling the end portions of the stator coil 16 in the front end. The external air is then expelled outside the case 3 through openings F.
In an automotive alternator of the above construction, external air flowing into the case 3 through the openings Al flows axially along the cylindrical wall 30a of the bearing housing portion 30 and then flows radially outwards, and one problem has been that the cooling efficiency for the positive-side diodes 23 has been poor because the amount of air striking the positive-side heat sink 24 is small and very little contributes to the cooling of the positive-side diodes 23.
Another problem has been that because the external air flows along flows axially along the curved cylindrical wall 30a of the bearing housing portion 30 and the surface area of the cylindrical wall 30a is small, the temperature of the first bearing 31 rises and the working life of the first bearing 31 is shortened.